1. Preheat the oven to 350°. Spread the diced bread on 3 rimmed baking sheets and bake just until crisp but not browned, about 15 minutes. Let cool, then transfer to a very large bowl.
2. In a small saucepan, bring 1/4 cup of the Madeira to a simmer. Add the currants, remove from the heat and let cool.
3. In a large skillet, melt 2 tablespoons of the butter. Add the sage leaves in a single layer and cook over moderate heat, undisturbed, until lightly browned on the bottom, about 2 minutes. Using tongs, turn each leaf and cook until crisp and brown, about 1 minute longer. Transfer to a plate to cool, then crumble coarsely. Add to the bowl with the bread.
4. In the same skillet, melt 2 tablespoons of the butter in 2 tablespoons of the olive oil. Add the garlic slices and cook over low heat, shaking the skillet a few times, until golden brown, about 3 minutes. Add the prosciutto and cook over moderate heat, stirring, until just starting to brown, about 2 minutes. Scrape into the bowl with the bread.
5. Melt 1 tablespoon of the butter in the remaining 2 tablespoons of olive oil. Add the onions, season with salt and pepper and cook over moderately high heat for 3 minutes. Reduce the heat to low; cook until softened, 20 minutes. Scrape the onions and fat into the bowl with the bread. Add the pears and the currants with their soaking liquid. Season with salt and pepper and stir well.
6. In a medium bowl, beat the eggs with 3 cups of the stock; add to the stuffing and stir well. Loosely fill the main turkey cavity with about 8 cups of the stuffing. Fill the neck cavity with about 2 cups of the stuffing and tuck the extra skin underneath or secure with toothpicks. Spread the remaining stuffing in a buttered large baking dish and refrigerate.
7. Preheat the oven to 425°. Set the turkey in a large roasting pan. Rub the turkey all over with the remaining 4 tablespoons of butter and season with salt and pepper. Fold the wing tips under so that the turkey sits upright. Add 2 cups of the Madeira and 1 cup of the stock to the roasting pan and roast the turkey for 30 minutes.
8. Baste the turkey and reduce the oven temperature to 350°. Roast the turkey for 1 hour, basting it after 30 minutes. Cover the turkey loosely with foil and roast for 1 hour longer, basting after 30 minutes. Reduce the oven temperature to 325° and roast for about 1 hour longer, or until an instant-read thermometer inserted in an inner thigh registers 170° and the stuffing registers 165°. Transfer the bird to a carving board and cover loosely with foil.
9. Increase the oven temperature to 400° and bake the stuffing in the baking dish for 20 minutes in the upper third of the oven, or until heated through and crisp on top.
10. Meanwhile, pour the juices in the roasting pan into a large glass measuring cup. Skim off the fat; add 3 tablespoons of the fat to a medium bowl and whisk in the flour. Pour the pan juices into a medium saucepan; add the remaining 5 cups of stock. Whisk 1 cup of the stock mixture into the flour paste, then whisk the mixture into the saucepan. Bring to a boil over moderately high heat, whisking constantly until thickened. Simmer over low heat, whisking occasionally, until no floury taste remains, about 10 minutes. Add the remaining 1 tablespoon of Madeira. Season the gravy with salt and pepper. Transfer to a warmed gravy boat. Carve the turkey and serve with the stuffing and gravy.
Make Ahead: The uncooked stuffing can be refrigerated overnight. Stuff the turkey just before roasting.
Wine Recommendation: A fine red Bordeaux or Napa Valley Cabernet blend will have the astringent tannins to check the richness here. Try the 1999 Château Gruaud Larose or the 1999 Napanook.